I had the same problem today. I have the dual version (not sure the tech term, but it runs two instances of Prime) on my old OS and just downloaded it on my new OS. I ended up with the single version (when you open the zip file, the Prime95 exe. logo is red. The dual version of it is green.
Anyway.....The green (dual) version you can select Round Off checking. The red (single) version it's grayed out. Now that your confused, hopefully someone else will explain it better.